T-T-T diagram of serpentine and saponite, and estimation ofmetamorphic heating degree of Antarctic carbonaceous chondrites

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For the estimation of peak thermal metamorphic degree of phyllosilicates in Antarctic carbonaceous chondrites, heating experiments of serpentine and saponite were carried out. A T-T-T diagram of serpentine for elevated temperature was obtained by heating the serpentine in the Murchison CM2 chondrite in vacuo at different temperatures (250∿1100℃) and for different durations (1 hour to 350 hours). Serpentine decomposed into olivine and enstatite through characteristic transitional structures. First decomposition started at about ∿300℃, the transitional structure appears at ca. 400∿500℃, and well-crystallized olivine and enstatite appeared above ∿750℃. In the T-T-T diagram, a minor time dependence was found. A T-T-T diagram of saponite was obtained in a similar was using terrestrial Fe-saponite. Saponite maintained its structure till about 800∿900℃, then decomposition started and finally enstatite was formed. New data on thermally affected phyllosilicates in Yamato (Y)-793321 indicate coexistence of 7Å unaltered serpentine and a transitional structure. Thermally affected saponite in Belgica (B)-7904 was also documented. Based on the T-T-T diagrams, the peak metamorphic degrees and peak temperatures were estimated in the order of B-7904≥Y-86720>Y-82162>Y-793321.
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